In industrial control systems, it is known to control an instrument, such as a valve controller, using a 4–20 mA DC signal supplied by a control system on a single pair of wires. Typically, the single pair of wires also provides electrical power to the instrument.
It is also known to superimpose bi-directional digital communications signals on the pair of wires. To achieve such communications, the instrument may include a variable impedance line interface circuit that maintains a low impedance at frequencies below 25 Hz to accommodate 4–20 mA analog signal variations without substantial terminal voltage fluctuation while also maintaining a substantially higher and relatively constant impedance across the frequency band (e.g., 500–5000 Hz) used for the digital communications.
The HART protocol is one known protocol for providing a 4–20 mA analog control signal in conjunction with bi-directional digital communications. The HART protocol achieves simultaneous analog and digital transmission by using a frequency shift keying (FSK) method to overlay a bi-directional digital signal on the analog control signal.